Photographic processes utilizing silver halide have heretofore been widely used due to excellent photographic characteristics, such as sensitivity, control of gradation, etc., as compared with other photographic processes, such as an electrophotographic process or a diazo photographic process. In recent years, a silver halide material-based technique capable of easily and quickly obtaining an image has been developed by changing the conventional wet development process using, for example, a developing solution, into a dry development process, such as a process using heat for development.
Heat-developable light-sensitive materials are known where this technique is used. Such heat-developable light-sensitive materials and processes therefor are described, for example, in Shashin Kogaku no Kiso, Corona Co., Ltd., pages 553-555, Eizo Joho, April 1978, page 40, Nebletts Handbook of Photography and Reprography, 4th Ed., Van Nostrand Reinhold Co., pages 32-33, U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020 and 3,457,075, British Pat. Nos. 1,131,108 and 1,167,777, and Research Disclosure, June 1978, pages 9-15 (RD-17029).
Many processes for obtaining color images using a dry process have been proposed.
With respect to processes for forming color images by the reaction of an oxidation product of a developing agent with a coupler, use of a p-phenylenediamine type reducing agent and a phenolic coupler or an active methylene coupler as described in U.S. Pat. No. 3,531,286, a p-aminophenol type reducing agent as described in U.S. Pat. No. 3,761,270, a sulfonamidophenol type reducing agent as described in Belgian Pat. No. 802,519 and Research Disclosure, pages 31 and 32 (September, 1975) and the combination of a sulfonamidophenol type reducing agent and a 4-equivalent coupler as described in U.S. Pat. No. 4,021,240, have been proposed.
These processes, however, are disadvantageous in that turbid color images are formed, because a reduced silver image and a color image are simultaneously formed in the exposed area after heat-development. In order to eliminate these disadvantages, a process which comprises removing the silver image by liquid processing or a process which comprises transferring only the dye to another layer, for example, a sheet having an image receiving layer has been proposed. However, the latter process is not desirable from the viewpoint that it is not easy to separate the dye from unreacted substances and transfer the dye only.
Another process, which comprises introducing a nitrogen-containing heterocyclic group into a dye, forming a silver salt, and releasing a dye by heat-development has been described in Research Disclosure, No. 16966, pages 54 to 58 (May, 1978). Clear images cannot be obtained using this process, because it is difficult to control the release of dyes from the unexposed areas, and thus it is not a generally applicable process.
Also, processes for forming a positive-working color image using a silver dye bleach process utilizing heat, with useful dyes and methods for bleaching have been described, for example, in Research Disclosure, No. 14433, pages 30 to 32 (April, 1976), ibid., No. 15227, pages 14 and 15 (December, 1976) and U.S. Pat. No. 4,235,957.
However, this process requires an additional step and an additional material for accelerating the bleaching of the dyes, for example, heating with a superposed sheet with an activating agent. Furthermore, this process is not desirable from the viewpoint that the resulting color images are gradually reduced and bleached by free silver which is present during long periods of storage.
Moreover, a process for forming a color image utilizing a leuco dye is described, for example, in U.S. Pat. Nos. 3,985,565 and 4,022,617. However, this process is not desirable because it is difficult to incorporate the leuco dye in the photographic material in a stable manner and coloration gradually occurs during storage.
Another disadvantage is that generally the above-described processes require quite a long time for development and result in images with high fog and low density.